The channel unit is well known for converting message and control signals exchanged between one channel of a multiplex carrier transmission facility and a telephone switching office communications line. All of the channel units that connect to the same carrier facility have the same carrier facility interface; however, the terminal unit interface of each channel unit changes as a function of the type of service and terminal unit.
Common message service channel units interconnect telephone offices that serve the public switched telephone network. These connections, called interoffice lines, interconnect two end switching offices, two transit offices, or an end office and a transit office.
Special service channel units usually interconnect a specific customer with an end office. In one application, a customer's station set, which would normally be served by a nearby local end office, is connected to a remote end office by an interoffice carrier channel line interconnecting the nearby and remote end offices. This foreign exchange service is commonly used for a business with a large community of customers in a neighboring city. The remote end office provides call progress control signals such as dial tone and audible ringing via the message path of the interoffice carrier line. However, the remote end must usually transmit information on a separate interoffice carrier line to ring the customer's station set.
In another special service application, a carrier facility line connects a private branch exchange (PBX) to a local end office switch, as with a large business customer. Here again, the end office supplies dial tone and audible ringing via the message path of the carrier line. Alternatively, the end office may send control information for the PBX to generate the various call progress control signals.
In most applications, the transmission facility as well as the channel unit must transmit certain control signals in addition to the message signal. These well-known control signals include supervisory, address, and call progress control signals and are used to oversee the dynamic nature of the traffic that is usually associated with the service.
Associated with the control signals on each communication line is a signaling protocol. This signaling protocol consists of those control signals that are exchanged on the line to originate a call and to advance the call from one supervisory state to another. Each control signal comprises, for example, a predetermined series, set, combination, or pattern of electrical interface signals. The d.c. electrical interface signals on each line typically represent the "on-hook/off-hook", "busy/idle", or "open/closed loop" condition of the line. However, it is the control signals or, more particularly, the various combinations or patterns of electrical interface signals, usually with respect to time, that are used to indicate the supervisory state of the call on the line, such as "idle", "seized", "answer", "disconnect", etc.
Prior art channel units simply convert the electrical interface signals of one line to the different electrical interface signals of another line. Prior art channel units do not recognize patterns of electrical interface signals. Thus, communication lines with different signaling protocols cannot be interconnected with prior art channel units. Previously, this interconnection could only be made through a switching office.
Connecting a carrier channel from a transit office directly to a PBX or a telephone station set while eliminating the end office interconnection creates numerous problems. First, a transit office, also referred to as a toll office, typically does not generate a complete set of necessary call progress control signals. Second, many types of PBX signaling protocols are incompatible with toll office signaling protocols. For example, a one-way incoming PBX line with immediate start operation does not return a wink supervisory signal that a one-way outgoing toll office line with a wink start operation expects to receive in response to seizing the PBX line. Third, toll offices generally do not respond to analog control signals such as dual tone multifrequency address signals. Lastly, prior art channel units convert only one set of electrical interface signals to another, thereby requiring a separately designed channel unit for each different type of terminal unit. Directly interconnecting two PBXs that have different signaling protocols presents even further problems, since neither PBX can convert one signaling protocol to another. Furthermore, directly interconnecting any two terminal units with a carrier facility presents similar problems when the signaling protocol of each unit is not compatible with the other. All of these problems are normally solved by the end office; however, without the end office, it would be extremely costly to retrofit toll offices or to develop individual interfaces for each terminal unit type to perform these end office functions.